Ni Layered Oxide as Electrode Material for Lithium

Article pubs.acs.org/JPCC

Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A 7Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors Anton Buzlukov,†,‡,§ Jean-Marie Mouesca,*,†,‡ Lucienne Buannic,∥,# Sabine Hediger,†,‡ Loïc Simonin,∥ Emmanuel Canevet,∥,⊥ Jean-Francois Colin,∥ Thibaut Gutel,∥ and Michel Bardet*,†,‡ †

Université Grenoble Alpes, INAC, SCIB/LRM, F-38000 Grenoble, France CEA, INAC, SCIB/LRM, F-38054 Grenoble, France § Institute of Metal Physics UB RAS, 18 S. Kovalevskaya Str., Ekaterinburg, Russia ∥ CEA, LITEN, DEHT/LCB, 38054 Grenoble, France ‡

S Supporting Information *

ABSTRACT: We present a 7Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/ discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO3 cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume ∼20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-like regions, most probably surrounding the Li2MnO3 domains and thus labeled “Ni-poor boundaries” (volume ∼21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (≤4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li1−xMO2-like domains via Ni2+ oxidation. At higher states of charge (≥4.5 V), the Li2MnO3-like domains become highly involved via oxygen-based (ir)reversible oxidation processes, leading to significant structural transformations. During discharge, only ∼60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMO2-like domains are fully refilled (via reversible Ni4+ reduction into Ni2+), while the ordered Li2MnO3-like domains experience a significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level.

1. INTRODUCTION The Li-rich Mn/Ni layered oxides combine high charge capacity (>240 mAh/g in the [2.0−4.6 V] range), low cost, safety, and good stability in time. They have thus attracted interest as one of the most promising active materials for positive electrodes in Liion batteries.1,2 However, during the first charge/discharge cycle, they exhibit an irreversible behavior which is not further observed for subsequent cycles (as for strategies to remedy the irreversibility in Li-rich Mn/Ni oxides, see Thackeray et al.3−5). Despite many research efforts, the mechanism of Li insertion−extraction and the reasons for both the high capacity and irreversibility character of this material were still not fully clear.6−13 By contrast, a better understanding of the oxygen© XXXX American Chemical Society

based reversible charge−discharge reactions in cathodic materials has been lately achieved.14−17 Even more recently, two joint papers, one experimental (P. G. Bruce et al.)18 and one theoretical (G. Ceder et al.),19 have appeared (see also the corresponding News & Views by C. Delmas)20 investigating the role played by oxygen in the reversible charge−discharge reactions within lithium-rich materials. As it turns out, oxygen environments resulting from overlithiation (i.e., of the OLi4M2type) are the ones prone to reversible oxidation. Received: July 27, 2016 Revised: August 4, 2016

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DOI: 10.1021/acs.jpcc.6b07532 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Electrochemical curves corresponding to the first cycle of Li1.2Mn0.61Ni0.18Mg0.01O2: Galvanostatic curve (a) and corresponding dq/dV curve with indication of the position of the studied ex situ samples (b).

code has been specifically developed for this purpose, and its results will be described. To ease our later discussion, it is reminded that the pristine material Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 belongs to a class of materials conventionally written as Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 (with x ≈ 0.18 in our case, neglecting Mg). There are, to first order, two borderline cases to describe its atomic arrangement. In one case, the homogeneous cationic distribution in the TM layer could form a solid solution as proposed by some authors.43−45 Alternatively, it can be viewed as a combination of monoclinic Li2MnO3 (C2/ m) domains within a trigonal LiMO2 (R-3m) matrix (where M are transition-metal elements).2,46 In such cases, the material can be formally decomposed into a mix of two ideal parent phases written as (2x)Li[Mn1/2Ni1/2]O2·(1−2x)Li[Li1/3Mn2/3]O2, with the two limits x = 0 (Li[Li1/3Mn2/3]O2 ≡ Li2MnO3) and x = 1/2 (Li[Mn1/2Ni1/2]O2) (see however McCalla’s critic of this view: Chapter 6, p.93 of ref 33). Phase separation into discrete domain structures, driven by dominant charge ordering, has been demonstrated for example in 0.5(Li2MnO3)·0.5(LiCoO2) by the combined use of HR-XRD, XAFS, NMR, and TEM data. We show here how local electroneutrality constraints allow us to derive from our 7Li MAS NMR data a similar phase separation.

Coming back to the observed high irreversibility, it is usually ascribed to the release of Li2O from the electrode material at high voltages.7,21−24 Oxygen loss from the surface is usually coupled with diffusion of transition-metal (TM) ions from the surface to the bulk where they can occupy the vacancies created by Li removal.7 As a result new phases are formed. Simonin et al. recently investigated the structural evolution of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 oxide.25−28 Measurement of the reciprocal magnetic susceptibility of the pristine material as a function of temperature revealed from its linear part a lower magnetic moment value (3.07 μB) than expected (i.e., 3.25 μB if only Ni2+ and Mn4+ are present).25 This had been then interpreted as the signature of the presence of Mn3+ ions (26% of the sample) in the pristine material. Alternatively, McCalla29−33 (see in his book:33 p.13, Chapter 8, p.111, and Chapter 10, p.135−136) explained the lowering of the magnetic moment by the presence of transition metal site vacancies. Moreover, in contradiction with previous literature, Simonin et al.25 observed the electrochemical activity of the Ni2+ ions for voltages higher than 3.9 V. Finally, the formation of a new spinellike phase was also reported at the highest states of charge.25 The present work follows previous investigations by Simonin et al.25−28 but from a different but complementary point of view. Namely, we performed 7Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments to study the structural changes in Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 during the first electrochemical cycle. NMR is a powerful technique which has been previously applied to study this type of materials.6,10,34−42 Most of these NMR studies were performed on 6Li-enriched samples, 6 Li having a smaller gyromagnetic ratio and quadrupolar moment values than 7Li, which leads to better resolved spectra. Since the main task of the present work is to trace the structural changes in the sample during both charge and discharge, i.e., extraction and reinsertion of lithium, the use of 6Li-enriched pristine material becomes inappropriate as 6Li would be replaced by 7Li during discharge. In addition, it can be expected that changes in the local environment of lithium will have a stronger effect on the 7Li NMR spectrum compared to the 6Li one due to the higher value of the 7Li gyromagnetic ratio. Taking these aspects into consideration, we concentrated our efforts on 7Li NMR spectroscopy. In order to strengthen our analyses and resolve some ambiguities that are still present in the literature, a local (i.e., bottom-up) NMR-dedicated model was developed to precisely determine all first and second cationic sphere configurations (including electroneutrality constraints) that can account for each 7Li signal experimentally recorded. A Fortran

2. EXPERIMENTAL DATA AND MODELING 2.1. Sample Preparation. A sample preparation procedure was previously described.25 Samples at various states of charge were obtained by cycling a composite powder containing 70% of active material and 30% of Super P carbon in a Swagelok cell. Around 30 mg was soaked in an electrolyte composed by 1 M LiPF6 in ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) in a 1:1:3 volume ratio. A glass fiber disk was used as a separator, and the counter electrode was a Li foil. The cycling rate was fixed at C/50 (one charge in 50 h). Figure 1 shows the first cycle obtained with this configuration. The cells were stopped at 3.9, 4.1, 4.3, 4.5, and 4.8 V during charge and at 4.1, 3.5, and 2.5 V during discharge. Those points correspond to either a local maximum or minimum of the dq/dV curve (Figure 1, right) and are thus delimiting different electrochemical processes. After cycling, the cells were dismounted in an Ar-filled glovebox. The cycled powder was washed in DMC and centrifuged 3 times to remove any remaining traces of electrolyte. The sample was then packed into the NMR rotor inside a glovebox. 2.2. Solid-State MAS NMR. Solid-state MAS NMR measurements were performed on a Bruker AVANCE DSX B


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The Journal of Physical Chemistry C 200 MHz spectrometer (4.7 T, 7Li Larmor frequency ν0 = 77.78 MHz) equipped with a 1.3 mm Bruker CPMAS probehead. The 7 Li MAS NMR spectra were recorded at different MAS spinning frequencies in the range 36−60 kHz with a rotor-synchronized Hahn echo pulse sequence (90° − τ − 180° − τ − acq, where τ = 1/νrot). Repetition delay for transient accumulation was 0.02 s. A 7 Li 90°-pulse width of 2.2 μs was used, corresponding to the nucleus magnetization turn angle of about 75−80°. Some spectra were recorded using the variable offset frequency technique: the carrier frequency was moved in steps of 80 kHz across the entire spectrum range, and the resulting spectra were mathematically added to yield the full spectrum. As no significant changes were observed compared to the corresponding spectra obtained with fixed carrier frequency (set in the middle of the spectrum), it was considered that the available Li rf-field strength was strong enough for a proper excitation of the entire spectrum width, and the carrier frequency was therefore fixed at 77.86 MHz. Aqueous solution of LiCl was used as a 0 ppm chemical-shift reference. All spectra were acquired with room-temperature bearing air, corresponding to a sample temperature in the range 50−70 °C. Fitting of spectra and extraction of NMR parameters (intensity, isotropic chemical shift, line width, and anisotropy of the dipole−dipole interaction tensor) were performed with DMFIT software.47 2.3. NMR-Dedicated Modeling and Selection of Cationic Configurations. The main idea of our NMRdedicated approach consists of exploiting the fact that 7Li chemical shifts are local probes sensitive to different configurations of paramagnetic TM ions among the 18 neighbors covalently linked to the central Li ion via oxygen bridge(s) and therefore located in the surrounding first and second cationic spheres (i.e., 12 × 90°(Li−O2−X) + 6 × 180°(Li−O−X), where X = Mn4+, Ni2+, Li+; cf. Figure 2), assuming its structural

therein). Were selected those configurations yielding a given experimental isotropic chemical shift within ±Δν/2 (expressed in ppm) to account for small variations in distances and bond angles (for example, ±50 ppm for line 1 centered at 776 ppm, etc.). In order to estimate isotropic chemical shifts for the theoretically possible configurations, hyperfine (Fermi contact) contributions of individual Mn4+ and Ni2+ ions were estimated based on our results and prior literature. Our only assumption at this stage will be that the pristine experimental lines 1 (776 ppm) and 2 (1556 ppm) correspond, respectively, to LiLi (i.e., Li belonging to the Li layers) and LiTM (i.e., Li belonging to the TM layers) environments in Li2MnO3 (i.e., with no Ni2+ among those 18 closest cations), in agreement with previous literature34−37,49 (see also refs 41 and 50). From known Li cationic environments,37 we derive chemical shift contributions of +259 ppm per (90°) Mn4+ (first cationic coordination sphere) and −65 ppm per (180°) Mn4+ (second cationic coordination sphere), in agreement with the literature.37 Moreover, from NMR studies of layered compound Li[Ni1/2Ti1/2]O2, Carlier et al.51,52 determined values for individual (90°) Li−O2−Ni and (180°) Li−O− Ni shift contributions: −10 ppm and +165 ppm, respectively. Considering these different contributions, we preselect combinations of Mn(90°), Mn(180°), Ni(90°), and Ni(180°) ions yielding experimentally observed NMR isotropic chemical shifts within ±Δν/2. 2.3.2. Local Electroneutrality. The electroneutrality at the level of the six oxygen anions A−F (cf. Figure 2) for the previously preselected configurations was checked. Pauling’s rule implies that, for each of the oxygen anions (O2−), the total cationic (partial) charge of the 6 octahedrally surrounding cations should be +2, resulting in a partial oxygen charge qi of zero. In the absence of oxygen vacancy and/or TM redox process, there are only a few possible cationic combinations neutralizing a given oxygen site, (Mn 4 + ) 2 (Li + ) 4 , (Mn4+)1(Ni2+)3(Li+)2, and (Ni2+)6, resulting from each other by the electroneutral substitution 1Mn4+ + 2Li+ ↔ 3Ni2+. Configurations selected during step (i) above do not necessarily satisfy Pauling’s rule at the level of each oxygen atom. We therefore computed for each preselected cationic configuration two numbers reflecting the deviation (if any) from strict electoneutrality, the sum of the six oxygen partial charges (Q = 6Σ1−6 qi), as well as a measure of the local deviations (DQ2 = 36Σ1−6 (qi)2) (factors 6 in Q and 36 in DQ2 are introduced for convenience to obtain integer numbers for Q and DQ2). For example, Q = 0 and DQ2 = 2 means charge separation with +1/6 and −1/6 on two oxygen anions, the four remaining oxygen partial charges being zero. For each of the five experimental lines, we selected those possible configurations having the lowest Q and DQ2 values, presuming them to be energetically the most stable ones. Both points i and ii combined lie at the heart of our selection process within our code. 2.3.3. Chemical Shift Anisotropies. Anisotropy Δδ (ppm) and asymmetry η parameters (Haeberlen’s convention;53−55 see also refs 36 and 41) were also computed assuming that the dipolar interaction to paramagnetic cations is the dominant contribution to our experimental chemical shift anisotropy. In a first approximation, all point-dipole contributions from magnetic ions of the first and second coordination spheres were added based on Strobel & Lambert-Andron’s crystal structure56 (see also refs 35 and 57). The resulting tensors were diagonalized, and the calculated Δδ and η values were compared to the experimental values extracted by deconvolution. It is clear of course that full dipolar shift tensors are not strictly local (i.e., they

Figure 2. Schematic representation and labeling of the 18 cationic sites surrounding a given central Li+ site. In our homemade Fortran code, the 12 (90°) Li−O2−X sites (weighing for 1/3) are labeled 1−6 in the intermediate (middle) layer labeled “90°-in” and 1−3 in the top layer and 4−6 in the bottom layer, both layers labeled “90°-out” (X = Mn4+, Ni2+, Li+). The six remaining (180°) Li−O−X sites (weighing for 1/6) are labeled I−III in the top layer and IV−VI in the bottom layer, both labeled “180°-out”. The six O2− anions are labeled A−F. This model amounts to six “full” cations.

integrity. Based on this model, we will introduce enough reasonable constraints to identify specific cationic configurations compatible with all NMR parameters for each of the detected NMR resonances. During the selection procedure, we searched to successively satisfy the four following constraints, all coded within a homemade Fortran program. 2.3.1. Isotropic Chemical Shifts. The central Li nuclear spin interacts additively with them via hyperfine Fermi-contact and dipolar interactions (see Middlemiss et al.48 and references C


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The Journal of Physical Chemistry C are not restricted to the first 18 cations’ contributions of Figure 2). For the 2b site of Li2MnO3 (i.e., our line 2), for example, Lee & Gray36 showed that adding point-dipole contributions beyond the first 6 Mn4+ ions of the TM layer reduces the computed Δδ value by about 20% (Lee & Gray’s Table 236). Beyond that, an additional empirical calibration was necessary (see details in the Appendix of the Supporting Information). The use of computed dipolar parameters was therefore tentative in our strategy, only to verify that specific cationic arrangements selected on the basis of points (i−ii) yielded dipolar parameters of the correct sign and order of magnitude. 2.3.4. Local Chemical Composition. We derived local chemical compositions in the normalized form (Li:Mn:Ni) = (a:b:c) (with a + b + c = 36, a, b, and c being integers) for each of the selected 18 cations’ configurations, taking into account the fact that each of the 12 (90°) Li−O2−X sites weighs for 1/3 and each of the 6 (180°) Li−O−X sites weighs for 1/6 (cf. Figure 2). This easy-to-read (a:b:c) formulation can be transformed into the LixMnyNizO2 equivalent formula via x = a/18, y = b/18, and z = c/18. For example, Li2MnO3 corresponds to (Li:Mn:Ni) = (24:12:0) and LiMn1/2Ni1/2O2 to (18:9:9). Our pristine material Li1.2Mn0.61Ni0.18Mg0.01O2 corresponds to the target composition ∼(21.7:11.0:3.3) (neglecting Mg). At a local level, all configurations preselected by the code after steps (i−iii) could actually exist within the pristine material (be it a solid solution or be it made of separate domains). Such an NMR-derived local 18cations’ composition (Li:Mn:Ni) could however also reflect the existence of a homogeneous domain of the same composition, in which case McCalla’s ternary (Li:Mn:Ni) phase diagrams29−33 can be used to set aside domains falling outside of the (cubic or layered) rocksalt boundaries (for which our structural model described by Figure 2 no longer applies).

Figure 3. 7Li MAS NMR spectra for pristine Li1.2Mn0.61Ni0.18Mg0.01O2 measured at MAS spinning speeds of 38 kHz (a) and 60 kHz (b) and the results of experimental spectra deconvolution. The isotropic line shifts are indicated, and all the other parameters of NMR lines remain the same at deconvolution for both spinning speeds (cf. Table 1).

3. RESULTS 3.1. Preliminary Analysis of the Solid-State NMR of the Pristine Material. Figure 3 shows the 7Li NMR spectra of pristine Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 recorded at 38 and 60 kHz MAS together with the result of their respective spectral deconvolution. The deconvolution was carried out with the simplest possible model, assuming that the NMR spectrum shape is defined by the hyperfine interactions (Fermi contact and dipolar) between the probed nuclei and the unpaired electrons of the transition metal cations.58 In contrast to 6Li, for which quadrupole interaction effects were found in processing the experimental NMR data for pure Li2MnO3 (see, e.g., ref 36), our attempts to account for quadrupole effects did not yield any noticeable changes of the spectral lines within the reasonable CQ range (up to 1000 kHz). This result is, in principle, quite expected taking into account the significantly higher value of the gyromagnetic ratio of 7Li compared to 6Li. As a result, electron− nuclear hyperfine interactions will play a much greater role in our case. Moreover, the bulk susceptibility can also be neglected as we used a 1.3 mm diameter rotor. Both 7Li spectra reveal the presence of at least five resonances including their respective spinning sideband (ssb) manifolds resulting from the modulation by MAS of the dipolar interaction of the Li spin with the time-averaged electronic magnetic moments of neighboring paramagnetic transition metal ions. For both spinning speeds, the deconvolution converged to the same NMR parameters for all five resonances, except for their isotropic chemical shifts. The shift to higher field observed from 38 to 60 kHz is related to an increase in sample temperature arising from additional frictional heating at higher MAS frequencies. Such a

temperature dependency is expected in paramagnetic materials and reflects the typical Curie−Weiss dependence of paramagnetic ions.37,58,59 The spectroscopic parameters extracted from the spectral deconvolution for each of the five resonances are reported in Table 1a−c with isotropic chemical shifts observed at 776 (line 1), 1556 (line 2), 576 (line 3), 1399 (line 4), and 946 (line 5) ppm for a spinning speed of 38 kHz. 3.2. Cationic Configurations Selected for the Pristine Material. In Table 2, cationic configurations compatible with both isotropic (δ) and anisotropic (Δδ, η) NMR parameters (i.e., satisfying points i to iii of our strategy, with Q = 0) are presented for experimental lines 1 to 5. For each selected configuration, alternative cationic arrangements yielding the same computed (isotropic and dipolar) NMR parameters were found. These arrangements result from global rotations around the central Li+ of the configurations reported in Table 2. As lines 1, 3, and 5 are expected to correspond to LiLi (due to Δδ ≥ 0)20 only the arrangements containing the maximal number of Li ions in the central [90°-in] layer of Figure 2 are reported in Table 2. For lines 2 and 4, tentatively assigned to LiTM (due to Δδ ≤ 0)20 arrangements containing the maximal number of Mn ions in the central 90°-in layer are chosen. All alternative arrangements, as well as additional configurations and corresponding arrangements yielding compatible isotropic shifts but discarded due to their dipolar parameters, are reported in Tables S1−5 and Figures S1−5. The effect of a global cationic rotation around the central Li ion is illustrated for line 1 in Figure S1b and for line 2 in Figure S2b. D


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Table 1. Absolute NMR Intensities % (Normalized to a Total of 100% For the Pristine Material), Isotropic Chemical Shifts δ (ppm), Line Widths Δν (kHz), and Dipolar Interaction Shift Anisotropy Δδ (ppm) and Asymmetry η for the Five Lines 1 to 5 (cf. Figures 3, 7, and 8)a line 1

a

sample

intensities (%)

δ (ppm)

pristine Uc = 3.9 Uc = 4.1 Uc = 4.3 Uc = 4.5 Uc = 4.8 -Ud = 4.1 Ud = 3.5 Ud = 2.5

44.2 39.8 57.9 63.4 40.5 13.4 15.9 16.1 21.8

776 771 771 768 726 646 -685 768 771

sample

intensities (%)

δ (ppm)

Δν (kHz)


20.3 20.3 11.7 14.8 6.1 0.0

576 574 575 574 551 --520 531 533

14.2 15.5 14.6 14.2 16.6 --29.8 29.5 26.2

4.2 15.6 24.2

Δν (kHz)

line 2 Δδ (kHz)

η

intensities (%)

δ (ppm)

132.7 129.2 122.4 126.7 99.8 78.2 -83.1 109.3 121.8

0.45 0.5 0.54 0.53 0.52 0.7 -0.66 0.5 0.55

14.1 8.2 9.7 11.8 7.8 1.4 2.3 3.7 8.2

1556 1545 1535 1521 1521 1521 -1550 1548 1551

Δδ (kHz)

η

intensities (%)

δ (ppm)

Δν (kHz)

Δδ (kHz)

η

126.1 129.2 113.8 125.1 97.2 --99.3 109.9 110.2

0.7 0.65 0.65 0.65 0.4 --0.63 0.72 0.7

9.8 11.7 8.1 0.0 0.0 0.0

1399 1379 1350 -----1370 1388

14.1 16.4 13.9 -----18.8 15.5

−163.9 −164.4 −164.4 -----−131.1 −130.4

0.2 0.1 0.1 -----0.51 0.8

7.8 8.1 10.1 10.3 18.0 28.3 -25.0 16.2 15.2 line 3

0.0 1.5 4.3 line 5

Δν (kHz)

Δδ (kHz)

η

−163.6 −164.4 −164.6 −163.6 −140.7 −120.7 -−129.9 −139.4 −140.2

0.2 0.2 0.15 0.1 0.2 0.2 -0.1 0.29 0.4

6.7 6.8 8.2 8.7 16.1 28.8 -21.6 15.7 14.8 line 4

sample

intensities (%)

δ (ppm)

Δν (kHz)

Δδ (kHz)

η


11.6 10.8 0.0 0.0 0.0 0.0 -0.0 0.0 0.0

946 914 ---------

14.1 17.1 ---------

110.6 110.2 ---------

0.1 0.1 ---------

Li content for each line at a given voltage value is obtained by multiplying its absolute NMR intensity as reported here by 1.2 (cf. Figure 1a).

Table 2. Configurations (All with Q = 0) Yielding the Five Targeted Experimental Isotropic NMR Signals 1 to 5a δ (ppm)

content

90°-in

90°-out

180°-out

elec.

exp.

computed

Li:Mn:Ni

Li

Mn

Ni

Li

Mn

Ni

Li

Mn

Ni

DQ2

line 1: 776 (LiLi) line 2: 1556 (LiTM) line 3: 576 (LiLi)

776 1554 538 587 1385 1436 996

24:12:0 24:12:0 16:8:12 16:8:12 22:11:3 18:9:9 22:11:3

6 0 4 3 0 0 5

0 6 0 0 5 4 0

0 0 2 3 1 2 1

2 6 1 0 6 5 2

4 0 2 3 0 0 4

0 0 3 3 0 1 0

2 6 0 4 4 2 2

4 0 4 2 1 1 3

0 0 2 0 1 3 1

0 0 2 2 2 2 2

line 4: 1399 (LiTM) line 5: 946 (LiLi)

The headings “90°-in”, “90°-out”, and “180°-out” refer to the cationic positions depicted in Figure 2. Also, chemical content (Li:Mn:Ni) as well as the local (DQ2) electroneutrality check (see main text) are given. Here are the only reported configurations compatible with experimental chemical shift anisotropic parameters (Δδ, η). Full data are given in the extended Tables S1−5. a

S1, and refs 34−37 and 49 (see also refs 41, 42, and 50)). The isotropic signal computed at 776 ppm yields configurations with five different (Δδ, η) NMR anisotropic parameters, but only one pair matches the experimental values (see Figure S1a left and S1b). Note that for this arrangement the central layer (here a Li

Cationic configurations found for each experimental line are as follows. 3.2.1. Line 1 (δexp = 776 ppm). The Fortran code recovers the typical Li-layer signal of the f ully electroneutral Li2MnO3 domain, i.e., (Li:Mn:Ni) = (24:12:0) (see the first line of Table 2, Table E


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Figure 4. Typical cationic arrangement yielding (a) best choice having δ = 538 ppm (DQ2 = 2) and computed dipolar terms: (Δδ, η) = (1222 ppm/95 kHz, 0.80) (exp: 1621 ppm/126 kHz and 0.70, respectively). Four Li(90°) are present in the central (Li) layer, as well as two Ni2+ ions. The top and bottom TM layers contain both Mn/Ni cations. (b) Second choice having δ = 587 ppm (DQ2 = 2) and computed dipolar terms: (Δδ, η) = (1163 ppm/ 90 kHz, 0.80). Li+ (small white circles), Ni2+ (small black circles), and Mn4+ (large black circles). Oxygen atoms are not represented.

Figure 5. Typical cationic arrangements yielding (a) δ = 1385 ppm and computed dipolar terms: (Δδ, η) = (−2071 ppm/−161 kHz, 0.16); (exp. −2112 ppm/−164 kHz, 0.2). Notice that five Mn(90°) are present in the central (TM) layer. (b) δ = 1436 ppm with (Δδ, η) = (−1710 ppm/−133 kHz, 0.17) are less compatible with experimental dipolar terms. Li+ (small white circles), Ni2+ (small black circles), and Mn4+ (large black circles). Oxygen atoms are not represented.

3 Ni2+ ions in its “Li layer” (see Figure 4b). It also exhibits three (180°) Ni−O−Ni pairs. 3.2.4. Line 4 (δexp = 1399 ppm). We found three computed isotropic shifts for line 4 (cf. Tables 2 and S4) with only two of them (1385 and 1436 ppm) satisfying points i−iii (another one computed at 1367 ppm has no dipolar match: see Table S4 and Figure S4a). Interestingly, the first one (δ = 1385 ppm) corresponds to a slightly Ni-contaminated Li2MnO3 structure, having (Li:Mn:Ni) = (22:11:3) but DQ2 = 2. It exhibits 5 Mn4+ ions in the TM layer, as anticipated in the literature52 (see Figures 5a and S4b), and therefore deviates only slightly from the Ni-free Li2MnO3 local structure. The other possibility (δ = 1436 ppm) has a maximum of 4 Mn4+ in the central (i.e., TM) layer and DQ2 = 2 (see Figure 5b). Although its composition (Li:Mn:Ni) = (18:9:9) is reminiscent of that of LiMn1/2Ni1/2O2, its TM layer cannot be extended into an ideal flower-type arrangement (see Figures S4c,d). Finally, these two possible (at this stage) configurations present no (180°) Ni−O−Ni motives. 3.2.5. Line 5 (δexp = 946 ppm). An ambiguity exists for line 5 as it had never been observed for layered materials before. A signal at around 950 ppm had been reported in the literature59 for a spinel cationic structure of composition LiMn3/2Ni1/2O4. No additional spinel phase has been observed experimentally for the pristine material (see Boulineau et al.).26,27 However, in a recent 7 Li NMR study of LiMn1/2Ni1/2O2 by Stoyanova et al.,42 the authors observed 7 isotropic lines (their Figure 6 and Table 2) among which one (measured at 958 ppm) is reminiscent of our line 5. We therefore considered that this line corresponds to a genuine rocksalt-layered arrangement. In that case (see Table 2 and Table S5), an isotropic chemical shift is preselected at 978 ppm with DQ2 = 0 (Table S5, top) with a very Ni-rich composition (12:6:18) which would put this first choice (as a

layer) of the local model of Figure 2 can be easily extended to a full layer of same symmetry (cf. Figure S1b, right). This longrange order is to be correlated with the fact that the line width of line 1 is two times narrower than that of lines 3−5. 3.2.2. Line 2 (δexp = 1556 ppm). As for line 1, the typical TMlayer signal of Li2MnO3 is recovered (see Table 2, Table S2, and refs 34−37 and 49 (see also refs 41, 42, and 50)). There are three possible (Δδ, η) pairs, with one matching the experimental values (Figure S2a, left). Similarly to line 1, the local order of the TM layer can be easily extended into a full layer (cf. Figure S2b), in agreement with the narrower signal recorded for line 2 in comparison to lines 3 and 5. 3.2.3. Line 3 (δexp = 576 ppm). Among the spectra reported in the literature for Li2MnO3−LiMO2 composites, lines 3 and 4 always appear together.6,38,49,52,60−63 A first isotropic shift is computed at 538 ppm having DQ2 = 0 (cf. the first two lines of Table S3 and Figure S3b). However, its unique set of computed dipolar parameters is not compatible with experiment, (Δδ, η) = (−946 ppm, 0.71), and it can be safely discarded (see Table 1). We therefore selected the next two configurations with DQ2 = 2 (δ = 538 and 587 ppm) in Table 2 based on dipolar terms (see Figure S3a top left and right). Both exhibit the same Ni-rich composition (Li:Mn:Ni) = (16:8:12), close to the (18:9:9) ≡ Li(Mn1/2Ni1/2)O2 phase. The first possibility (δ = 538 ppm, DQ2 = 2) has 4 Li+ cations and 2 Ni2+ ions in its “Li layer” (see Figures 4a and S3c). More interestingly, it exhibits three (180°) Ni−O− Ni bonding motives, known to stabilize the energy due to antiferromagnetic coupling.64 This may partly compensate for the energy cost induced by charge separation (DQ2 = 2) and account for the relatively high intensity of line 3 (22% of the total intensity) which will be discussed later (Section 5). The second Ni-rich possibility (δ = 587 ppm, DQ2 = 2) has 3 Li+ cations and F


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point has the following average composition: (14.8:6.1:15.1) (±0.7), reasonably close to our (16:8:12) selected for NMR line 3. To conclude with the pristine material, we showed that NMR signatures reveal the existence of two types of domains. One type will be called Ni-poor Li2MnO3-like and combines (Ni-free) lines 1−2 within the cores and lines 4−5 as Ni-contaminated “boundaries”, of average composition (23.45:11.73:0.82) (computed from Table S6a) that is Li1.30Mn0.65Ni0.05O2 (cf. Section 2.3.42.3 iv) of volume: 73% (Li content: 80%). Another one will be called Ni-rich LiMn1/2Ni1/2O2-like corresponding to line 3 of composition (16:8:12) that is Li0.89Mn0.44Ni0.67O2 of volume 27% (Li content: 20%). This last NMR line 3 is thus the clear signature of the drive of the pristine material toward Ni clustering, a drive confirmed by Stoyanova et al.’s measurements.42 One can anticipate the existence of two antagonistic driving forces, one toward the remarkably stable Li2MnO3 domains (even if including a few Ni ions at the “boundaries”) and the other one toward Ni segregation and, therefore, Ni-rich domains close to LiMn1/2Ni1/2O2 (more about this in the Discussion, Section 5). Interestingly, Boulineau et al.27 (see also ref 26) combined different spectroscopic techniques (XRD, XAS, TEM, etc.) to analyze the same pristine material and arrived at the following composition for the Li-rich/Ni-poor domains: Li1.27Mn0.60Ni0.13O2 (equivalently: (22.9:10.8:2.3)). This composition is indeed intermediate between (24:12:0) (lines 1−2) and (22:11:3) (lines 4−5). It is therefore in good agreement with our own NMR-derived formula. The only experimental difference is that they probe a local region of their material, whereas NMR probes the whole bulk sample. Boulineau et al.27 also derived a composition for the Li-poor/Ni-rich domains (i.e., Li1.0Mn0.74Ni0.26O2) which differs from ours (Li0.89Mn0.44Ni0.67O2 being Mn-poorer/Ni-richer). Still, their relative weights (70 and 30%, respectively) are compatible with ours (80 and 20%, respectively).

phase) in the cubic rocksalt region. Moreover, this configuration of high local symmetry (see Figure S5b) has no dipolar parameters (Δδ, η) matching the experimental values (cf. Figure S5a top left). Another possible candidate with DQ2 = 2, computed at 996 ppm with a local composition (22:11:3), mirrors that found for one of the selected arrangement matching line 4 (see Tables 2 and S4). It has a particularly good dipolar match (cf. Figure S5a top right) and is represented in Figure 6 as our best choice. Again, as for line 4, this configuration does not present any stabilizing (180°) Ni−O−Ni motif.

Figure 6. Typical cationic arrangements yielding δ = 996 ppm with (Δδ, η) = (1334 ppm/104 kHz, 0.08) otherwise compatible with experimental dipolar terms (1425 ppm/111 kHz, 0.1).

3.3. Discussion of the Results from the NMR-Dedicated Model. A general comment is in order before coming back to each experimental line. First, all selected solutions reported in Table 2 exhibit configurations with Q = 0 (i.e., global neutrality of the 18 peripheral cations’ model of Figure 2). In other words, all chemical formulations (Li:Mn:Ni) reported in Table 2 and Tables S1−5 derive from (24:12:0) (≡Li2MnO3) by the electroneutral substitution 1 Mn4+ + 2 Li+ ↔ 3 Ni2+ (cf. Tables 2 and S1−5). In Figure S6 is presented a simplified version of McCall’s ternary (Li:Mn:Ni) phase diagram (cf. Chapter 5, Figure 5.2 of ref 33). It can be seen that a selected (Li:Mn:Ni) composition can exist as an extended domain with hexagonal layered structure from (24:12:0) (Ni-free) to (14:7:15) (Nirich). For higher Ni content, the material falls into the cubic rocksalt domain. A final step in our analysis consists in testing the hypothesis that the pristine material is organized in chemically homogeneous nanoscale subdomains. For that, we combine various (Li:Mn:Ni) compositions selected for lines 1−5 and weigh them appropriately to compare the outcome with the target pristine composition ∼(21.7:11.0:3.3). Technical details have been reported in Table S6a. The best solution (see Table S6b) is made of a majority domain: (24:12:0) (≡Li2MnO3-like) from lines 1 and 2 (i.e., Ni-free within the 18 cations’ model of Figure 2). The remaining material splits into two subdomains: Ni-rich (16:8:12) for line 3 and Ni-poor (22:11:3) for lines 4−5 combined. Our best proposal is remarkable as it can be directly related to McCalla’s results. 29−33 As it turns out, depending on experimental conditions (heating temperature and subsequent quenching versus slow cooling, etc.), the domains containing Ni ions (lines 3−5) can turn out unstable, separating into two sublayered structures called N and M by McCalla33 (see Chapter 6, Figure 6.5). The M point in the phase diagram has the following average composition (in our notation): (21.6:11.5:2.9) (±0.7) (derived from Table 6.2 of ref 33), virtually identical to our (22:11:3) selected for NMR lines 4−5 combined! The N

4. EX SITU NMR ANALYSIS OF ELECTROCHEMICALLY CYCLED MATERIALS Figure 7 presents the 7Li NMR spectra acquired at 38 kHz MAS of the pristine sample and its evolution during charge and discharge at different cutoff voltages (at Uc = 4.8 V, the intensity is twice enlarged for better visualization). In order to compare the spectra, their signal intensities have been normalized by both the amount of sample in the rotor and added transients. To quantify the spectral changes as a function of the state of charge, deconvolution has been applied to all spectra of Figure 7. The extracted parameters are gathered in Table 1, and the evolution of lines 1−3 as a function of the state of charge and discharge is shown in Figure 8a−c. For charged samples, the spectral resolution allows the identification of all NMR resonances identified in the pristine material and the evaluation of their characteristics. However, for discharged samples, broadening of the NMR resonances prevents clear identification. As a result, and in particular for the samples at Ud = 3.5 and 2.5 V, the NMR spectra could be satisfyingly described by only four lines. Nevertheless a minor contribution of the fifth resonance cannot be excluded. Before discussing the behavior of each individual NMR line during charge and discharge, let us first compare the evolution of Li content during charge and discharge measured by NMR to that derived from electrochemistry (Figure 9). First of all, both electrochemical and NMR-based cycle shapes are quite similar, G


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Figure 7. 7Li MAS NMR spectra for pristine Li1.2Mn0.61Ni0.18Mg0.01O2 and for charged/discharged samples with different cutoff voltages, obtained at 38 kHz spinning speed by using rotor-synchronized echo pulse sequence. Two isotropic positions at 776 and 1556 ppm are shown. The NMR signal intensities were normalized on number of acquisitions and mass of active material filling rotor. For the highest charge state the NMR signal intensity was increased twice for simplification of presentation.

Figure 8. Evolution of 7Li NMR spectra parameters for layered LixMn0.61Ni0.18Mg0.01O2 oxides during electrochemical cycle. There are shown the values of isotropic shifts, δ (a), widths, Δν (b), and anisotropies of dipolar interaction tensor, Δδ, (c) for line 1 (filled signs) and line 2 (open signs) for the pristine sample (squares) and charged (right triangles) and discharged (left triangles) samples. The corresponding parameters for line 3 are shown as squares for the pristine sample and as open and filled circles for charged and discharged states, respectively. The solid lines are shown as a guide to the eye.

illustrating the fact that the NMR experiment as performed here can be considered as quantitative. This is especially true for the first part of charge [3.5−4.3 V] where both curves basically coincide (taking into account ±5% errors for NMR Li contents). For higher charge voltage, however (≥4.5 V), as well as during discharge, there is a shift between the two curves, the NMR total intensity (and therefore total Li content in the material) being systematically lower than that derived from electrochemistry (∼0.19 Li starting with Uc = 4.5 V and for the rest of the cycle). Two nonmutually exclusive explanations can be proposed at this stage. First, a similar discrepancy between NMR and electrochemistry-derived lithium counts has been previously reported for static NMR experiments65 (see Figures 2 and 4 there). This discrepancy was assigned to signal decay before acquisition, due to the presence of transition metals (more specifically Mn3+) inducing rapid T2 relaxation of 7Li nuclei. However, MAS NMR experiments are expected to reduce to some extent such fast T2 relaxation problems due to elongation of the NMR signal decay and line narrowing. Alternatively, the ∼0.19 Li content’s difference during discharge could putatively be related to (nonredox) processes resulting in a net loss of Li2O from the Li2MnO3 domains rewritten for that occasion as Li2O−MnO2 (without Mn4+ redox change).2,7,13 In such a case, electrochemistry would not count such extracted Li ions, whereas NMR counts all Li ions present in the material at a given voltage. In the absence of a clear explanation of the origin of this 0.19 Li difference, we will refer to it as “(nonredox) Li2O loss” from now on.

Figure 9. Total Li content derived from NMR lines’ intensities (charge: circles; discharge: squares). Dotted line: report of the first electrochemical cycle’s data (cf. Figure 1).

Changes along the charge and discharge cycle occur at two (interconnected) levels: electronic (redox or not) and/or structural. On the redox side, two main families of mechanisms can be invoked.14,15,22,43,66−68 A first one involves TM (Ni2+ and/or Mn3+ if any are already present in the pristine material or created during the last part of discharge, below 3.5 V). This would imply oxidation (upon charge) and reduction (upon discharge). A second mechanism involves oxygen (partial) oxidation up to irreversible O2 release.7,22,69 From a structural H


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(represented by lines 4 and 5) of initial Li content 0.26. Therefore, not all Li from these Ni-poor regions will be extracted by Ni2+ oxidation: 0.14 Li (about half) remains, later extracted by the same (oxygen-based) processes invoked later for Li2MnO3 domains above 4.3 V (see below). However, line 4 already disappeared after 4.1 V and line 5 after 3.9 V: what does happen to their NMR signatures after Ni2+ oxidation? To answer this question, let us turn to lines 1−2. 4.1.2. Lines 1−2 (i.e., Ni-Free Sites). In terms of intensities, up to 4.3 V, one would expect line 1 and 2’s intensities to vary little as they belong to Li2MnO3-like domains, while those of lines 4−5 would decrease (partly due to Ni2+ oxidation, thus following line 3: cf. above). However, when plotting line 1’s Li content as a function of charge (cf. Figure 10a, where the NMR-based intensities of the five lines 1−5 are converted to Li content and plotted, from the data reported in Table 1), a curious phenomenon is observed, as the (Li-layer) line 1 experiences a sudden and unexpected increase of intensity (during delithiation!) between Uc = 3.9 and 4.1 V reaching 0.69 (+0.16 compared to initial Li content: 0.53). It can be seen however that, between the same two voltage values (Li-layer), line 5 suddenly disappears (drop of 0.13 Li). A tentative explanation can be proposed. In fact, both lines 1 (Ni-free; cf. Figure S1b) and 5 (Ni-poor; cf. Figure 6) correspond to Li+ cations within Li layers and share very similar cationic environments. In particular, for line 5 (measured at 946 ppm: Table 1), there are 4 × Mn(90°) and 3 × Mn(180°) contributing up to 845 ppm to line 5′s chemical shift. During charge (but before reaching the plateau), the (paramagnetic) Ni2+ cations surrounding the Li+ cation yielding line 5 are fully oxidized into (diamagnetic) Ni4+ leaving there 0.14 Li (cf. above). The point is that about half of the initial Li+ ions is not extracted yet, and once Ni2+ ions are magnetically extinguished by oxidation, only Mn4+ ions will determine their (paramagnetic) chemical shifts, therefore expected around 845 ppm for most of the remaining Li+ ions, close to that of line 1 (776 ppm). The apparently correlated behaviors of both (Li-layers) line 1 (increase) and line 5 (decrease) between Uc = 3.9 and 4.1 V could therefore be interpreted in this crude scenario as a “conversion” of part of line 5 into “1-like” features, added on top of the original Li2MnO3-like line 1 not yet affected by delithiation at that voltage (cf. Figure S8). The same reasoning could be expected for part of line 4 (cf. Figure 5a) converting into a “2-like” line (the other half of 0.14 Li) in the same voltage range. First, it can be seen that line 2’s intensity increases from 8.0% = 0.1 Li at 3.9 V up to 10.4% (0.12 Li) at 4.3 V, but the Li site yielding line 4 is surrounded within its TM layer by 5 × Mn(90°) (+ one diamagnetic Ni4+) and by one additional Mn4+ cation lying within the nearby Li layer (cf. Figure 5a). An isoelectronic Ni4+ ↔ Mn4+ permutation driven by the formation of a particularly stable planar LiMn6 configuration would yield a “2-like” line. In terms of their δ, Δν, and δΔ values, lines 1 and 2 clearly follow two regimes (Figure 8). Up to Uc = 4.3 V, the widths (Δν) and the anisotropies of dipolar interaction (Δδ) of both lines exhibit only slight changes, indicating the absence of significant structural changes in the Li2MnO3 domains up to a charge state of 4.3 V. This allows us first to assume that the previously observed changes in the lattice parameters25 are mainly associated with structural changes in the Li1−xMO2 matrix and second to conclude that the size of these Li2MnO3 domains is large enough to not be affected by the loss of cations in surrounding Ni-containing LiMO2-like fields, thus keeping the

point of view (mainly translated into NMR line broadening and Δδ value changes), TM migration within the material7 concomitant with irreversible oxygen loss would have to be considered. We now discuss the behavior of the NMR lines, during charge and then during discharge. Charge/Delithiation. 4.1.1. Lines 3−5 (i.e., Ni-Containing Sites). The intensity of the Ni-rich LiMO2-like line 3 steadily decreases (within experimental error) over the 3.5−4.5 V charge voltage range as expected for an NMR line signaling the presence of Ni-rich domains (cf. Figure 10a). Moreover, line 3 presents

Figure 10. (a) Absolute NMR Li contents of lines 1−5 upon charge (cf. Table 1). (b) Absolute NMR Li contents of lines 1−4 upon discharge (cf. Table 1).

only one regime in terms of the behavior of spectrum parameters, with no significant changes of δ, Δν, and δΔ values upon charge at least up to Uc = 4.3 V (see Table 1 and Figure 8). This indicates that the Li+ cations detected by NMR and still present in the Li1−xMO2 matrix during delithiation are not structurally affected by the electrochemical process. This does however not mean that the delithiated sites are not structurally damaged. The behavior of line 3 evidences Ni2+ oxidation, as Mn4+ oxidation into Mn5+ does not occur (Mn5+ in octahedral coordination is not feasible). In between 4.3 and 4.5 V, all Ni2+ are expected to be fully oxidized into Ni4+ (Ni2+/3+/4+ oxidation starts around 3.7 V and is fully reached around 4.45 V).13 This redox mechanism translates into the global equation Li1.2Mn 0.61(Ni 2 +)0.18 O2 → Li 0.84Mn 0.61(Ni4 +)0.18 O2 + 0.36Li+ + 0.36e−

The initial Li content of the Ni-rich domains as measured by the initial NMR intensity of line 3 is 0.24 Li (i.e., 20.3% × 1.2). The rest (i.e., 0.12) comes from Ni-poor (Li2MnO3-like) regions I


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capacity of Li-rich layered oxides. In that case, one would have (from the same starting point as above, i.e. after Ni full oxidation)

average structure close to the original. On the other hand, the isotropic shift behavior for lines 1 and 2 is highly differentiated: while the δ value for line 1 remains constant, the isotropic shift of line 2 clearly decreases. This is most likely due to the release of a few Li+ from TM layers to the Li layers (deduced from Figure 11

Li 0.84Mn 0.61(Ni4 +)0.18 (O−2 )2 → Li 0.84 − 2zMn 0.61(Ni4 +)0.18 (O−2 + z )2 + 2z Li+ + 2z e−

The length of the plateau determined by (redox-based) electrochemistry corresponds to 0.84−0.33 = 0.51 Li (compared to ∼0.86−0.18 = 0.68 by NMR), that is 2z ≈ 0.51. Notice however that (O−2+z)2 can be rewritten in order to make visible the irreversible loss of molecular O2 Li 0.84Mn 0.61(Ni4 +)0.18 (O−2 )2 → Li 0.84 − 2z ′Mn 0.61(Ni4 +)0.18 (O−2 )2 − z + 2z′Li+ + 2z′e− + (z′/2)O2

As far as redox processes are concerned, reality would be a mix of the two previous equations (hence z + z′ = 0.255), and the discharge behavior may offer us a way to quantify both of them. Finally, the Li content corresponding to line 1 starts to drop significantly from 0.76 at 4.3 V down to 0.16 at 4.8 V (from 0.14 to 0.02 for line 2) for a total delithiation of 0.72 Li. At the end of the charging process, the Li layers of the Li2MnO3-like domains are not fully emptied (it remains about 0.16 Li), whereas the TM layer is almost empty (0.02 Li). This result coincides with quantum calculations performed by Xiao et al.70 4.1.3. More General Remarks. At the final charge voltage (4.8 V), the 7Li NMR spectrum becomes suddenly broader (Figure 7), with more symmetric spinning sidebands manifold. It consists mainly of the 1-like line centered at 646 ppm and some 2-like signal at δ = 1521 ppm with low (of about 1−2%) total intensity (Table 1). Our data are in qualitative agreement with previous results for the related Mn/Ni layered oxides, which showed a broad featureless spectrum at the highest states of charge.10,38,63 Nevertheless, the isotropic shift of the literature spectra is at about 550 ppm, which is usually associated with lithium ions in the Li layer coordinated by Mn4+ and Ni4+ ions. The δ values closer to our 646 ppm for line 1 were previously observed for layered compounds with LiMnO2 at highly deintercalated states.75 Our results would confirm our assumption that, for highest states of charge, the Li ions that still remain in the sample are concentrated exclusively in the Li2−xMnO3 domains, whereas the Ni-containing domains are already empty at Uc > 4.5 V. However, similar δ values were also observed for some spinel compounds, in particular for the Li x Mn 2 O 4 at highly deintercalated states.76 Thus, our results could alternatively evidence the formation of a new spinel-like phase at this charge voltage as was assumed in some papers.11,12,25−27 The real situation could however be more complicated as it was shown by Armstrong et al.75 and Reed et al.77 that the transformation from layered to spinel structure can be performed without change of the anions’ sublattice via migration of 25% of Mn ions into octahedral positions of the Li layer, combined with the displacement of Li ions into tetrahedral sites. As mentioned by others,75 the local environment of lithium ions in such tetrahedral sites is very similar to that found for lithium ions in tetrahedral sites of ordered Mn(IV) spinel LiZn0.5Mn1.5O4. The isotropic shift and the spinning sideband manifold of our spectrum (which is close to being symmetrically distributed around the isotropic resonance) are indeed similar to those found for LiZn0.5Mn1.5O4.36 Thus, we could assign our NMR signals centered at 646 and 1521 ppm to Li ions placed in highly

Figure 11. Ratio of line 2 over line 1 intensities during charge (circles) and discharge (squares). Dashed line: expected 1/3 ratio for Li2MnO3 domains.

plotting line 2 over line 1 intensities’ ratio). This leads to distortions of Mn hexagons which are reflected in the δ value of the remaining LiTM (line 2). The observed higher rate of Li extraction from TM layers is consistent with the results of quantum calculations70 and coincide in general with the assumptions that Li ions in the TM layer move first into the Li layer before being extracted from the bulk through the lithium planes. When Uc ≥ 4.5 V, the NMR parameters for lines 1 and 2 change drastically (see Table 1). They exhibit an increase of Δν and decrease of Δδ values (in modulus) (see Figure 8b and c). This indicates that, first, the Li2MnO3 domains are highly involved in Li deintercalation processes at these Uc values, and second, the remaining Li ions reside in an increasingly distorted crystalline environment (most likely located near the domain “boundaries”), which is due both to local displacements of Mn4+ ions and anion loss (for more details on structural changes, see refs 4, 5, 71, and 72). Indeed, the decrease of modulus of Δδ suggests that the local crystal structure around the remaining Li ions is no longer purely layered but closer to cubic. As for the mechanism(s) involved upon charge above Uc = 4.5 V, once lithium has been removed from the LiMO2 component (i.e., our line 3) by Ni2+ oxidation, Li2O could be irreversibly extracted above 4.5 V from Li2MnO3 as already stated above.2,7,13 Applying this to what is left from our material after full Ni oxidation, one would have Li 0.84Mn 0.61(Ni4 +)0.18 O2 → Li 0.84 − 2yMn 0.61(Ni4 +)0.18 O2 − y + y Li 2O (loss)

This (nonredox) Li2O extraction mechanism would allow to explain the difference of 0.15 Li observed between NMR (0.18 Li) and electrochemistry (0.33 Li), in which case 2y ≈ 0.15. In a series of papers, Delmas et al.14,15,73 argue for the concomitance of both reversible and irreversible processes. The reversible process corresponds to 2O2− → O2n− (1 < n < 3) (cf. also Sathiya et al.17,74) occurring in the Li2MnO3 component. This would now proceed without oxygen loss and minimal structural changes and would be in part responsible for the extra J


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The Journal of Physical Chemistry C distorted sites (octa- or tetrahedral) in the “former” Li layer of Li2MnO3 and to those blocked in the sites within the TM layer, respectively. Finally, with 0.18 Li left as measured by NMR, we have Li0.18Mn0.61(Ni4+)0.18(O−1.67)2. From Lu & Dahn’s expression22 (see their Table 1) with our Ni concentration x = 0.18, one would predict: Li0.21Mn0.61(Ni4+)0.18(O−1.68)2 very close to the NMR expression. Discharge/Relithiation. 4.2.1. Lines 3−5 (i.e., Ni-Containing Sites). We plot in Figure 10b the NMR Li contents as a function of discharge voltage Ud. The first striking observation has to do with the fact that line 3 (i.e., the Ni-rich LiMO2-like domains) is the fastest one to recover its initial pristine content (steepest slope, within experimental accuracy), and the redox process appears to be fully reversible in the LiMO2 domains as the previously fully oxidized Ni4+ cations are now fully reduced back to Ni2+. The final intensity of line 3 (27.1%) being close to (even larger than) the initial one (20.3%) gives the indication of a fully reversible charge/discharge process in the LiMO2-like domains in terms of Li content (Table 1 and Figure 12 plotting

increasingly more ions are placed back into their pristine positions in Li and TM layers within Li2MnO3 domains, as illustrated for example by the plot of line 2 over line 1 intensities’ ratio (Figure 11). Interestingly, while the chemical shift values of lines 1 and 2 almost reach their initial values at the final discharge state (Ud = 2.5 V), Δν and Δδ are rather different from those observed for the pristine sample (Figure 8). We attribute these effects to the influence of structural disorder within the second and higher coordination spheres, arising most likely from transition metal ions being blocked in the lithium layers during the Li reinsertion process6 and preventing Li diffusion in the bulk part of these domains. In the two-composition notation, this means a decreased size of the Li2MnO3 domains after the first charge/ discharge cycle. In the pristine material, the intensity ratio of line 3 over lines 1 + 2 (i.e., the size of the LiMO2 versus Li2MnO3 domains) is close to the expected value (0.422) computed assuming full phase separation (the boundary case represented by lines 4−5 is discussed below) but reaches at the end of discharge a much higher value due to a significant intensity loss of lines 1 + 2 (the intensity of line 3 is fully recovered: see Figure 10). 4.2.3. More General Remarks. At this stage of the discussion on discharge, no Mn4+ reduction into Mn3+ has been invoked. It is difficult to state what would be the corresponding NMR signal when looking at the loss of resolution of the NMR spectrum at the end of discharge, with Mn3+(90°) ≈ −16 ppm and Mn3+(180°) ≈ 33 ppm.37 At this stage, looking for cationic configurations involving Li+, Mn4+/Mn3+, and Ni2+ is beyond the scope of the present work. Concerning the Mn3+ presence, the “best” way to solve this question (i.e., Simonin et al. vs MacCalla) would be to try to perform direct zero-field 55Mn NMR measurements.78 From a quantitative point of view, we do not recover all Li cations during discharge, but about the same amount is reinserted whether measured by NMR (0.70−0.18 = 0.52 Li) or electrochemistry (0.93−0.33 = 0.60 Li). By electrochemistry, 0.36 Li are accounted for by reversible Ni4+ reduction to Ni2+ leaving therefore 0.60−0.36 = 0.24 Li for reversible oxygen reduction. Moreover, the total irreversible loss would be 1.20− 0.93 = 0.27, assuming that this is fully due to irreversible oxygen oxidation (combined with related or unrelated blocked sites and other effects preventing Li diffusion). Interestingly, upon investigation of the electrochemical activity of pure Li2MnO3 by measuring the concomitant Li extraction and molecular O2 production, Yu et al.69 observed that, indeed, the amount of oxygen gas accounts for half of the charge compensation, as estimated here.

Figure 12. Ratio of line 3 over line 1 + 2 intensities during charge (circles) and discharge (squares). Dashed horizontal line: expected ratio for a full (2x)LiMO2−(1−2x)Li4/3Mn2/3O2 phase separation, computed as (2x)/((1 − 2x)(4/3)) with x = 0.18 (Ni content). Dotted line: ratio of line 3 over lines 1 + 2 + 4 + 5 intensities for the sake of comparison (see main text: quantitative analysis).

line 3 over lines 1 + 2 intensities’ ratio). At the end of discharge, line 3 accounts for 0.33 Li, very close (within experimental accuracy) to the expected 0.36. Thus, the Ni redox-based relithiation of the Ni-rich LiMO2 domains provides a sufficiently strong driving force to overcome any hindrance coming from crystal structure damages that arose during Li extraction. NMR reveals that all line 3 spectral parameters are modified, and a strong increase in line broadening can be noticed. This suggests that Li+ ions reinserted into the Ni-rich LiMO2-like domains experience a more disordered local cationic mix compared to the pristine material. Finally, we observed that the Ni-poor Li2MnO3-like domains (lines 4 and 5) quickly disappear during charge with no detectable intensity left above 4.3 V. At the end of discharge (Ud = 2.5 V) only line 4 is detectably recovered with final spectral parameters comparable to their initial values (although the final spectrum at 2.5 V is poorly resolved, the intensity of this line is small, and its parameter values can have large errors). 4.2.2. Lines 1−2 (i.e., Ni-Free Sites). During discharge, the δ, Δν, and Δδ values of lines 1−2 (i.e., the Li2MnO3 domains) tend to revert back toward their initial values observed for pristine material (Figure 8a−c). This indicates that during Li reinsertion

5. DISCUSSION The 7Li NMR study of a promising active material for the positive electrode in lithium batteries, the Li1.2Mn0.61Ni0.18Mg0.01O2 layered oxide, was carried out ex situ during the first charge/ discharge reaction. In order to assign the five lines experimentally detected by NMR (Figure 3), we developed an NMR-dedicated code assuming a local layered/rocksalt structure and selecting all 18-cation configurations (cf. Figure 2) compatible with experimental (isotropic and anisotropic shift) NMR parameters (cf. Table 1). This homemade code relies on taking into account local electroneutrality and yields local chemical compositions expressed as (Li:Mn:Ni) normalized to a sum of 36 cations. Comparing the NMR parameters extracted from deconvolution to their computed values allowed us to conclude that the pristine K


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But, as evidenced for our line 3, this will create local violations of the electroneutrality rule. For our lines 3−5, we found cationic configurations having Q = 0 but ΔQ2 = 2, meaning that two oxygen anions (among the six of Figure 2) have nonzero local formal charges (qi = +1/6 and qj = −1/6). The cost for such a charge separation (i.e., for creating a dipole) can be roughly estimated by a point-dipole model yielding the energy80,81 E = 14.4 × (2q2/(2rO) − q2/dOO) where rO is the ionic radius of oxygen (i.e., 1.26 Å); dOO is the oxygen interdistance (2.8 Å); and q2 = (qi)2 = (qj)2. Numerically: E ∼ 0.1 eV. This validates a posteriori our strategy of choosing cationic configurations exhibiting as small ΔQ2 values as possible (while satisfying other NMR-related constraints) as the energetic cost for nonzero ΔQ2 is rapidly high. For our pristine material, due to the competition between electroneutrality and antiferromagnetism, the Ni-rich LiMO2-like domains will exhibit frustration, the outcome of which being in our case an average composition around our (16:8:12) in the Ni-rich domains. Regarding the behavior of the material during the first charge/ discharge cycle, let us state that at the initial states of charge (up to the cutoff voltage equals to 4.3 V) Li+ ion extraction is nonuniform and occurs mainly from the Ni-rich Li1−xMO2-like domains and only partly from the Ni-poor (i.e., (22:11:3)) domains. To all appearances, the remaining lithium sites in these last Ni-poor domains are “converted” into NMR signals very similar to lines 1−2 of Ni-free Li2MnO3 due to great structural similarity. At higher states of charge (4.5−4.8 V), the Li2−xMnO3like domains become highly involved in the Li-ion deintercalation process, as evidenced by line broadening. The Li ions are released from TM layers slightly faster than from the Li layers (see Figure 11 and Table 1). Oxygen (ir)reversible mechanisms are involved (up to Li2O and O2 gas loss). At these high voltages, structural transformations most probably lead to a spinel-like phase formation and the trapping of the residual Li ions in tetrahedral sites located between TM and Li layers.71 During discharge, the reinsertion of lithium into the structure was clearly observed by NMR. Resonances corresponding to the Ni-rich LiMO2-like matrix in the fully discharged samples are composed of a slightly shifted line 3 (now at 533 ppm) corresponding to domains less ordered than in the pristine material (doubled line width). The intensity (i.e., Li content) of this signal 3 is recovered due to reversible Ni redox processes. One also recovers both signals 1−2 corresponding to Li2MnO3 and with doubled line widths. However, their intensities combined (∼0.36 Li) are only about half of its original value (∼0.7 Li), an observation related to the fact that the activation of the Li2MnO3 domains is caused in part by irreversible oxygen oxidation resulting into the loss in the form of O2 gas. This implies a global decrease of the size of Li2MnO3-like domains with respect to the Ni-rich LiMO2-like matrix after the first cycle.

material consists of highly ordered Ni-poor Li2MnO3-like domains (combining our lines 1−2: ∼58% and 4−5: ∼21%, for a total of ∼80%) joined to Ni-rich LiMO2-like domains, characterized (within the limit of NMR sensitivity) by one single line around 576 ppm at 38 kHz (line 3 of Figure 3a, counting for ∼20%). In comparison a strict two-phase formulation of the material into “pure” Ni-free Li 2 MnO 3 and Ni-rich LiMn1/2Ni1/2O2 would have led to 62% of the former and 38% of the latter. Some Ni contamination of the (Ni-free) Li2MnO3 domains therefore occurs at the “boundaries” between Ni-free and Ni-rich domains, giving rise to the two distinct lines 4 and 5, both with (Li:Mn:Ni) = (22:11:3). We recover by NMR McCalla’s phase separation into ternary points M ∼ (22:11:3) and N ∼ (16:8:12)33 (see Chapter 6, Figure 6.5). Next, we can reframe our work on the pristine material ∼ Li1.2Mn0.61Ni0.18O2 within the wider context of the Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 family. Let us start with the fact that the composition proposed for our line 3 at 576 ppm (16:8:12) is close to that of LiMn1/2Ni1/2O2 ≡ (18:9:9) studied by Stoyanova et al.42 Interestingly, the dominant 7Li NMR line they measured is indeed the equivalent signal at 533 ppm (42.5%, for the same magnetic field 4.70 T but higher spinning rate 90 kHz: see Table 2 of ref 42). The intensity pertaining to the Nirich line 3 (at 576 ppm and equivalent) is therefore doubled from our pristine material to the Ni-rich LiMn1/2Ni1/2O2 (18:9:9). In the process, new signals appear at 298 and 859 ppm reported by Stoyanova et al.42 and identified as very Ni rich by our NMRdedicated program: (12:6:18) and (10:5:21), respectively (see Table S7 and Figure S7). Stoyanova et al.’s line at 859 ppm (relative intensity: 14.4%) is the second line in terms of Ni content after the line at 533 ppm (relative intensity: 42.5%). These two Ni-rich lines account for most of the Ni cations of LiMn1/2Ni1/2O2. Finally, it can be verified that if the total Li content of the Ni-rich domains keeps increasing as expected (from 0% to 22% to 61% for Li2MnO3, our pristine data and Stoyanova et al.’s data, respectively) that of the Ni-poor “boundaries” apparently reaches a maximum (from 0% to 21% to 12%, respectively). This shows that layered material derived from Li2MnO3 can only accommodate a certain percentage of Ni contamination before aggregation starts to form Ni-rich domains. Notice next that LiLi NMR resonances (for example our line 3) could be maintained close to its present isotropic value in spite of further Ni insertion into the Li layer (cf. Figure 5 of Hinuma et al.67) since the paramagnetic shift contribution to LiLi of each Ni(90°) within the Li layer is small (∼−10 ppm). Still, a gradual shift toward smaller δ values and line broadening should be ultimately observed. The tendency for Ni2+ densification (i.e., drive toward NiO) interpreted in light of our NMR-dedicated model has already been reported in the literature and is explained as resulting from two antagonistic effects. Van der Ven and Ceder64 (see also Hinuma et al.79) verified computationally that the flower-like structure initially proposed for LiMn1/2Ni1/2O2 is further stabilized (via Ni ↔ Li exchange) when Ni ions in the Li layer form (180°) NiLi−O−NiTM bonds with Ni in the TM layer, due to the stabilizing antiferromagnetic superexchange mechanism operating within such bonds. We have estimated by DFT that each such (180°) NiLi−O−NiTM motif stabilizes the local structure by ∼0.01 eV. Therefore, it can be expected that the higher the number of LiLi−O−Ni2+ bonds there will be before Ni ↔ Li exchange, the more likely this will cause the replacement of LiLi by Ni2+ to form as many NiLi−O−NiTM bonds as possible.

6. CONCLUSION We have shown how 7Li NMR-MAS experiments can help answering questions about the fine structure of Li[Li(1−2x)/3Mn(2−x)/3Nix]O2 materials in favor (in our case) of a separation into (nanoscale) subdomains.2,13,82 This is at first counterintuitive as our NMR experiments only probe locally (cf. Figure 2) the cationic environment of a given lithium ion. However, combining experimental δ values (five in our case) and local electroneutrality is sufficient to select only a handful of cationic configurations: either Ni-free/-poor (close to Li2MnO3) or Ni-rich (close to LiMn1/2Ni1/2O2) with nothing detectable in between. Obviously, it would be interesting to pursue with 7Li L


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(6) Breger, J.; Meng, Y. S.; Hinuma, Y.; Kumar, S.; Kang, K.; ShaoHorn, Y.; Ceder, G.; Grey, C. P. Effect of high voltage on the structure and electrochemistry of LiNi0.5Mn0.5O2: A joint experimental and theoretical study. Chem. Mater. 2006, 18, 4768−4781. (7) Armstrong, A. R.; Holzapfel, M.; Novak, P.; Johnson, C. S.; Kang, S.-H.; Thackeray, M. M.; Bruce, P. G. Demonstrating oxygen loss and associated structural reorganization in the lithium battery cathode Li[Ni0.2Li0.2Mn0.6]O-2. J. Am. Chem. Soc. 2006, 128, 8694−8698. (8) Lei, C. H.; Bareno, J.; Wen, J. G.; Petrov, I.; Kang, S. H.; Abraham, D. P. Local structure and composition studies of Li1.2Ni0.2Mn0.6O2 by analytical electron microscopy. J. Power Sources 2008, 178, 422−433. (9) Tran, N.; Croguennec, L.; Menetrier, M.; Weill, F.; Biensan, P.; Jordy, C.; Delmas, C. Mechanisms associated with the ″Plateau″ observed at high voltage for the overlithiated Li(1.12) (Ni(0.425)Mn(0.425)Co(0.15))(0.88)O(2) system. Chem. Mater. 2008, 20, 4815−4825. (10) Jiang, M.; Key, B.; Meng, Y. S.; Grey, C. P. Electrochemical and Structural Study of the Layered, ″Li-Excess″ Lithium-Ion Battery Electrode Material Li[Li1/9Ni1/3Mn5/9]O-2. Chem. Mater. 2009, 21, 2733−2745. (11) Hong, J.; Seo, D. H.; Kim, S. W.; Gwon, H.; Oh, S. T.; Kang, K. Structural evolution of layered Li1.2Ni0.2Mn0.6O2 upon electrochemical cycling in a Li rechargeable battery. J. Mater. Chem. 2010, 20, 10179−10186. (12) Ito, A.; Shoda, K.; Sato, Y.; Hatano, M.; Horie, H.; Ohsawa, Y. Direct observation of the partial formation of a framework structure for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O-2 upon the first charge and discharge. J. Power Sources 2011, 196, 4785− 4790. (13) Croy, J. R.; Kim, D.; Balasubramanian, M.; Gallagher, K.; Kang, S.H.; Thackeray, M. M. Countering the Voltage Decay in High Capacity xLi(2)MnO(3)center dot(1-x)LiMO2 Electrodes (M = Mn, Ni, Co) for Li+-Ion Batteries. J. Electrochem. Soc. 2012, 159, A781−A790. (14) Koga, H.; Croguennec, L.; Menetrier, M.; Mannessiez, P.; Weill, F.; Delmas, C. Different oxygen redox participation for bulk and surface: A possible global explanation for the cycling mechanism of Li1.20Mn0.54CO0.13Ni0.13O2. J. Power Sources 2013, 236, 250−258. (15) Koga, H.; Croguennec, L.; Menetrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C. Reversible Oxygen Participation to the Redox Processes Revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 2013, 160, A786−A792. (16) Koga, H.; Croguennec, L.; Menetrier, M.; Mannessiez, P.; Weill, F.; Delmas, C.; Belin, S. Operando X-ray Absorption Study of the Redox Processes Involved upon Cycling of the Li-Rich Layered Oxide Li1.20Mn0.54Co0.13Ni0.13O2 in Li Ion Batteries. J. Phys. Chem. C 2014, 118, 5700−5709. (17) Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C. P.; Vezin, H.; Sougrati, M. T.; Doublet, M. L.; Foix, D.; Gonbeau, D.; Walker, W.; et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. Nat. Mater. 2013, 12, 827−835. (18) Luo, K.; Roberts, M. R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y.-S.; Edstrom, K.; Guo, J.; Chadwick, A. V.; Duda, L. C.; et al. Chargecompensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nat. Chem. 2016, 8, 684−691. (19) Seo, D.-H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials. Nat. Chem. 2016, 8, 692− 7. (20) Delmas, C. Battery Materials: Operating through oxygen. Nat. Chem. 2016, 8, 641−643. (21) Lu, Z. H.; MacNeil, D. D.; Dahn, J. R. Layered Li[NixCo1− 2xMnx]O-2 cathode materials for lithium-ion batteries. Electrochem. Solid-State Lett. 2001, 4, A200−A203. (22) Lu, Z.; Dahn, J. R. Understanding the anomalous capacity of Li/ Li[Ni xLi (1/3−2x/3)Mn (2/3-x/3)]O 2 cells using in situ X-ray diffraction and electrochemical studies. J. Electrochem. Soc. 2002, 149, A815−22.

solid-state MAS NMR this task by varying the amount of nickel to cover the whole range from x = 0 (i.e., Li2MnO3) to x = 0.5 (cf. Stoyanova et al.).42 Our pristine-based analysis has been confirmed by following lithium extraction and reinsertion in a very detailed way, and the NMR-derived cycle has been compared to the corresponding charge/discharge cycle. In the near future, we will apply our methodology to follow by NMR such first cycles by varying the nickel content (as done by electrochemistry)22,28,43,66,67 in order to further characterize and optimize such materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07532. Procedure for dipolar modeling (Annex A), additional cationic configurations for lines 1−5 for the pristine material (Tables S1−5, Figures S1−5), simplified (Li:Mn:Ni) phase diagram (Figure S6), combinations of (Li:Mn:Ni) compositions (Tables S6a,b), cationic configurations predicted for LiMn1/2Ni1/2O2 (Table S7 and Figure S7), and Li content of lines 1 + 5, 2 + 4, and 3 as a function of charge (Figure S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: (33) 4 38 78 57 72. Fax: (33) 4 38 78 50 90. *E-mail: [email protected]. Tel.: (33) 4 38 78 30 13. Fax: (33) 4 38 78 50 90. Present Addresses

(L.B.) CIC Energigune, Parque Tecnológico de Á lava, Albert Einstein, 48. Edificio CIC, 01510 Miñano, Á lava. ⊥ (E.C.) Laboratory for Neutron Scattering and Imaging, Paul Scherrer Insitut, CH-5232 Villigen PSI, Switzerland. #

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ALB is grateful for the financial support of FASO of Russia (theme N° 01201463330).

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REFERENCES

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